straightlight's already mentioned it, but Geo IP Tools can do this. You can block users by geo zone or IP address, and they won't have access to any pages in your store. They'll just see a "blocked" message, which you can set in the extension. It works with all OpenCart versions from 1.5.1.x through 3.0.2.0.

Feel free to take a look at the screenshots and demo site, and if you're interested let me know at www.getclearthinking.com/contact if you have any further questions.